DESCRIPTION: The investigators propose to continue studies on the mechanisms of sickle cell adhesion to endothelial cells of the microvasculature, as further described by their abstract: "Polymerization of hemoglobin SS at low oxygen tension is assumed to be the dominant factor in sickle cell pathology. Since morphologic sickling is delayed after hemoglobin deoxygenation, factors which slow red cell microcirculatory transit are likely antecedents to microvascular occlusion, ischemic tissue damage, and pain episodes characteristic of sickle cell anemia. We hypothesize that sickle erythrocyte adherence to microvascular endothelium delays erythrocyte microcirculatory transit to initiate or propagate vaso-occlusion. Our studies have identified plasma factors, red cell receptors, and endothelial cell adhesion molecules which promote sickle cell adherence under flow. In vivo, adherence will have to occur under dynamic conditions and adhesion must be strong enough to withstand shear forces exerted by flowing blood to reperfuse occluded vessels and tissues. Thus, receptors, cell adhesion molecules, and adhesive proteins are necessary, but not sufficient for sickle cell adherence and vaso-occlusion. We hypothesize that in vivo, adherence biophysics (primarily strength) will regulate the extent of sickle cell adhesion and vaso-occlusion. Conditions which lead to extensive and strong adherence will be most relevant to the pathophysiology of sickle cell adherence and vaso-occlusion in vivo. To address this, experiments have been designed with the following specific aims: (1) Demonstrate that sickle cell adherence under flow is stronger than adherence under static conditions and that adherence under flow selects for cells expressing more adhesion receptors; (2) Quantify the strength of sickle-cell/endothelial adherence in response to red cell agonists, endothelial activators, and adhesive plasma proteins for cell populations and individual cells in the range of 0 - 10 dynes/cm2 shear stress. Particular emphasis will be placed on conditions which invoke multiple adherence pathways to mimic clinical conditions which may precipitate pain episodes in vivo, such as thrombosis or infection; (3) Develop flow channels with venular dimensions (20-50 micrometers) and demonstrate that sickle cell adherence in confined channels is strong. "Analysis of the biophysics of sickle cell adherence to endothelium under conditions which mimic those in the microcirculation will provide insights into the physiological conditions which promote extensive, rapid, and strong sickle cell/endothelial cell adherence likely leading to vaso-occlusive pain episodes. These studies will identify the most important parameters and adherence pathways to target for the development of therapeutic agents to inhibit or reverse adherence to ameliorate sickle cell pain episodes."